1. Field of the Invention
The present invention relates to a transponder, an interrogator, and a communication system in which an interrogator transmits a carrier wave to a transponder and the transponder receives the carrier wave, modulates the received carrier wave, and returns the modulated carrier wave as a reflected wave to the interrogator.
2. Discussion of Related Art
Japanese Patent Publication No. 2000-49656 or its corresponding U.S. Pat. No. 6,177,861 discloses a wireless communication system including a plurality of transponders and an interrogator that transmits a carrier wave to the transponders so that each of the transponders modulates the carrier wave using an information signal such as an identification (ID) signal identifying the each transponder, or using a subcarrier wave that has been modulated using the information signal, and returns the modulated carrier wave as a reflected wave to the interrogator.
However, in the case where the communication system employs a great number of transponders, it is difficult, in view of the signal reception and demodulation capacity of the interrogator, to adapt all the transponders to use respective subcarrier waves having different frequencies. Thus, there has been a problem that the total number of subcarrier-wave frequencies that can be used by the transponders is limited and accordingly the probability that the subcarrier waves may interfere with each other is increased. In addition, in the case where each transponder is adapted to be able to change the frequency of subcarrier wave used thereby, the each transponder cannot know the subcarrier-wave frequencies used by the other transponders and accordingly cannot avoid the possibility of interference of subcarrier waves. Moreover, in the case where the communication system employs a plurality of interrogators and those interrogators are adapted to transmit respective carrier waves having different frequencies, each transponder does not have the function of selecting a specific frequency and accordingly modulates and returns all the carrier waves transmitted from the interrogators. Thus, each interrogator cannot avoid receiving the modulated carrier waves returned from all the transponders including other transponders than a target transponder and accordingly the possibility of interference of modulated carrier waves is increased.